This invention relates to a lock and, more particularly, to an electronic digital combination lock for preventing access to an enclosure without the correct combination.
Electronic combination locks are currently in wide commercial use to control access to protected areas. These locks eliminate the need for a key and with it the problems associated with loss, theft or duplication of the keys. Access is gained to the protected area when the correct combination is entered into the lock, whereby the lock will be opened.
In one type of electronic digital combination lock, a panel of push buttons is mounted on a wall near a door, outside the protected area, while an electronic control box is mounted on the wall on the inside of the protected area. The panel may have ten numbered buttons and by pressing, for example, four buttons in proper sequence corresponding to the combination, a circuit in the control box will be activated to energize a solenoid of an electric door strike to allow the door to be opened.
Such a push button combination lock suffers from a number of disadvantages. After a short period of use of depressing the four buttons of the combination, they will show some wear that will at least partially reveal the combination. Also, installation of the electronic push button lock is relatively expensive since there is required the running of multiconductor cables between the panel and the control box and between the control box and the door strike, as well as the mounting of the panel, box and door strike on the wall. This installation also requires defacing of the wall to accomplish the needed mounting and running of the cables when, for example, an existing mechanical lock is being replaced with the electronic lock. In addition to the installation costs, the push button panel, the control box and the door strike are relatively expensive items, thereby adding to the cost of the lock. Furthermore, the electrical power requirement of this type of electronic lock is so great that the power source is typically an existing 115 volt AC supply, with battery operation being used only for emergency standby in the case of power failure. The need for this battery also increases the cost of the lock.
In another type of electronic digital combination lock, particularly that shown in U.S. Pat. No. 4,019,355, a set of push buttons is exposed on the surface of a door knob or handle. The lock is opened by simultaneously depressing one or more of the push buttons for each digit in a multi-digit combination and rotating the door handle while the buttons are depressed for each digit. When the correct combination is entered, an electromagnet is energized to permit the door handle to be rotated sufficiently to retract a bolt out of a mating recess in an adjacent door jamb for access to the protected area.
While the above patented electronic combination lock has the advantages of utilizing an electronic circuit requiring low power consumption so that it is battery operated, and of not requiring the mounting of a push button panel on a wall adjacent the door, it has disadvantages which make it unsuitable for commercial use. Manipulation of this patented lock is relatively difficult since it requires depressing the push buttons while simultaneously rotating the door handle to open the lock. Also, in addition to the push buttons, the electronic circuitry requires the use of a relatively large number of physical switches mounted internally on or in relation to the door handle to open the lock, together with a relatively complicated logic circuit that would not make it compatible for production on an inexpensive integrated circuit board in view of the many circuit connections that would be required for connection to the board. Still further, the mechanical structure of the patented lock constitutes a complete departure from prior mechanical locks. Consequently, it does not have the desirable feature of being a relatively simple variation of an existing lock, which would, for example, make for reduced costs in design and manufacturing.